* {
  margin: 0;
  padding: 0;
}

.box {
  display: flex;
  justify-content: center;
  align-items: center;
}

.boxA {
  width: 400px;
  height: 400px;
  background-color: red;
}

.boxA .boxB {
  width: 300px;
  height: 300px;
  background-color: green;
}

.boxA .boxB .boxC {
  width: 200px;
  height: 200px;
  background-color: khaki;
}
